These studies will examine the molecular mechanisms for repair of radiation-induced DNA damage in human tissue culture cells. The project focuses on determining whether ionizing and ultraviolet radiation may induce gene expression. Although DNA damage inducible functions have been identified and extensively studied in E. coli (the SOS system), the evidence for inducible functions in mammalian cells is incomplete and largely based on induction, mutagenesis and host cell reactivation of viral probes. We propose to detect and clone genes inducible by ionizing or UV radiation by using two sensitive and direct techniques: 1) detection of radiation induced alterations in specific mRNA contnent using hybridization of cDNAs against a genomic library, and 2) induction of selectable markers fused into eukaryotic transcriptional units via transfection. Identification and cloning of inducible genes will lead to further elucidation of the pathways for repair of radiation-induced DNA damage in eukaryotes. This may have broad implications in mutagenesis and carcinogenesis.